Current limiting fuses have long been used in the protection of electrical distribution systems. Generally, these fuses comprise a fusible element of varying cross section surrounded by finely divided refractory material.
Current limiting fuses are extremely well suited for interrupting high range fault currents inasmuch as such fuses significantly reduce energy dissipation during fault clearance. In advanced designs, a silver fusible element having a plurality of zones of reduced cross section is employed, the zones of the element being rapidly vaporized upon experiencing a fault current such that arcing occurs across each of the spaces formerly occupied by the reduced cross section zones. This multiple arcing continues at a rapid rate until sufficient arc resistence is generated to present aggregate voltage drops exceeding the normal system voltage whereupon the fuse current is forced to zero long before the peak value of the available fault current is obtained. The refractory material surrounding the silver element forms a glassy matrix which entraps the silver vapor in order to preclude restriking of the arc.
The primary advantage of current limiting fuses over other fuse devices is their ability to limit energy dissipation and contain all energy of interruption thereby operating in a totally nonviolent manner even in response to very high fault currents. Thus, current limiting fuses offer effective highly dependable protection against the deleterious effect of high fault currents. Moreover, current limiting fuses may be safely and advantageously used in areas where violent fuse operation would prove annoying or even hazardous.
One problem common in even the most advanced current limiting fuse designs is that of providing reliable protection against a full range of fault currents. In this regard, current limiting fuses inherently generate arc resistance at a rate which increases with the magnitude of the fault current; consequently, high range fault currents are interrupted more rapidly than low range fault currents. This characteristic presents a design dilema inasmuch as the fuse must be capable of generating arc resistence slowly enough to avoid generation of excessive peak arc voltages under high fault current conditions, while at the same time the fuse must be able to generate arc resistence fast enough under low fault conditions to interrupt the fault before the supply of refractory material is exhausted by heating from the arc. These competing opposed design considerations have seriously frustrated attempts to produce a full range current limiting fuse.
Numerous efforts have been made to develope a true full range current limiting fuse, the simplest being the use of so called "M spot" elements. The "M spot" elements are hybrids, comprising a conventional silver element provided with a section of low melting point metal for the purpose of interrupting low fault currents. However, current limiting fuses of this design are subject to thermal degradation sometimes resulting in melting of the element and subsequent arcing at a current below the rated current of the fuse, thereby permitting sustained arcing and ultimate rupture of the fuse housing. Of course, such operation is simply not acceptable in view of the service demands made upon today's utility companies.
A more sophisticated approach to the above mentioned problem is disclosed in U.S. Pat. No. 3,304,387 issued to Lindell and entitled "Current Limiting Fuse Having Parallel Current Limiting Elements In A Series Connected Current Calibrated Element With Auxiliary Arc Gaps To Blow The Current Limiting Elements One By One". The Lindell device employs a number of parallel current limiting elements designed to collectively carry the rated current of the fuse and effectively interrupt high magnitude fault currents, and a means for distributing the full fuse current to each element individually upon encountering a low magnitude fault current. Lindell accomplishes current distribution by the provision of a current calibrated fusible element connected in series with the current limiting elements through extensions on each respective element. A series of arc gaps, one for each current limiting element, is coupled in parallel with the fusible element in order to provide sequential distribution of current to the individual current limiting elements upon fusing of the fusible elements and extensions. In this manner, the current limiting elements are successively fused under the influence of the low fault current until the fault current is cleared upon fusing of the last current limiting element. One problem with this device is that of extinguishing the rather stable single arc, formed upon fusing of the fusible element, in order to initiate sequential arcing of the gaps in the current limiting elements. Lindell proposes several different mechanisms in an attempt to accomplish satisfactory extinguishment of the initially formed single arc as in U.S. Pat. Nos. 3,304,388, 3,304,389, and 3,304,390. However, none of these devices has proved entirely satisfactory and it is believed that the inability to provide a means for effectively and reliably extinguishing this initial arc has prevented successful commercialization of the Lindell fuse.